The human retina is susceptible to damage from a variety of environmental factors, including laser light impact and other trauma, as well as disease. Once damaged, the cells responsible for capturing light energy and transducing it into a neural signal, the photoreceptors, do not regenerate. In fact, none of the neural cells of the retina can yet be made to readily regenerate in the adult human. When damage is severe enough, there is permanent vision loss in an area. Healthy photoreceptors do not migrate long distances toward the damaged area to replace damaged ones.
If the affected region is in the central macula, known as the fovea, then the ability to see fine detail, read at rapid rates, or recognize objects at great distances may be lost. The peripheral areas of vision do not have sufficient sampling density to perform these tasks to the same degree. Thus, early detection and treatment of potentially sight-robbing damage are crucial in maintaining central vision.
One of the chief problems in early detection of damage has been the difficulty of imaging a small area of retina. The macula presents a small target—6000 microns. The portion that is necessary for seeing damage that precludes observation of fine detail and reading is even smaller, about 600 microns. To examine this latter portion properly, it is desirable to image the central 20 degrees of the macula with sufficient magnification and contrast to determine whether an individual is at risk for permanent vision loss.
The opthalmoscope or fundus camera has traditionally been used to view and image the retina. Originally, these devices flooded the retina with white light. Subsequent devices have used selective wavelengths that have been found suitable for viewing or imaging particular structures or contrast between structures. Regardless of the wavelength of light used, many of the past devices used flood illumination, producing images of the retina that often are subject to poor contrast due to long range scatter. The long range scatter problem was identified to occur, not only from out of plane tissues, but also from the biological tissues that are inherently scattering, especially those within and near the retina.
One well-known method of reducing the long range scatter problem is to replace a flood illumination source with a scanning illumination source. Some research has suggested that the use of a double scanning optical apparatus that scans both incident and reflected light using a horizontal scanning element may be desirable. Scanning with such an element can be performed by a rotating multifaceted polygonal reflector, and a vertical scanning element, such as a reflecting galvanometer. Such an instrument is able to provide a two-dimensional output representative of reflection characteristics of the eye fundus. See, e.g., U.S. Pat. Nos. 4,768,873 and 4,764,005, as well as U.S. Pat. No. 4,768,874 each disclosing a laser scanning opthalmoscope in which a line beam is scanned across an eye. Such improvements have greatly increased the contrast of the images produced, but typically require expensive, heavy equipment that must be operated by a specialist.
Improvements on the scanning illumination source technology have been embodied in the use of advanced reflectometry techniques with a scanning laser opthalmoscope (“SLO”) as developed by the inventor, Ann Elsner, and colleagues. See, for example, Elsner A. E., et al., Reflectometry with a Scanning Laser Opthalmoscope, Applied Optics, Vol. 31, No. 19 (July 1992), pp. 3697-3710 (incorporated herein by reference). The SLO is advantageous for quantitative imaging in that a spot illumination is scanned in a raster pattern over the fundus, improving image contrast significantly over flood illumination. The inventor's SLO technology can further eliminate unwanted scattered light by using confocal apertures such as a circle of variable diameter or annular apertures, depending on the desired mode. Once the light is returned through the confocal aperture, the desired light can then be transmitted to a detector. However, the optics used in confocal apertures can increase the complexity of the system, and high quality optics are an added expense. Therefore, a method for reducing or eliminating unwanted scattered light in a more cost effective manner would be greatly appreciated.
Further improvements to increase contrast in retinal imaging systems include the extensive use of near infrared light as an illumination source, in lieu of other wavelengths or color images, as developed by the inventor and colleagues and described in Elsner, A. E., et al., Infrared Imaging of Sub-retinal Structures in the Human Ocular Fundus, Vision Res., Vol. 36, No. 1 (1996), pp. 191-205; Elsner, A. E., et al., Multiply Scattered Light Tomography: Vertical Cavity Surface Emitting Laser Array Used for Imaging Subretinal Structures, Lasers and Light in Opthalmology, 1998; Hartnett, M. E. and Elsner, A. E., Characteristics of Exudative Age-related Macular Degeneration Determined In Vivo with Confocal and Indirect Infrared Imaging, Opthalmology, Vol. 103, No. 1 (January 1996), pp. 58-71; and Hartnett, M. E., et al., Deep Retinal Vascular Anomalous Complexes in Advanced Age-related Macular Degeneration, Opthalmology, Vol. 103, No. 12 (December 1996), pp. 2042-2053 (all of which are incorporated by reference herein). Combining infrared imaging with SLO allows the use of reflectometry techniques to view the eye rapidly and noninvasively because infrared light is absorbed less than visible light and scatters over longer distances. Further, when implemented with scanning laser devices, infrared and near infrared imaging of sub-retinal structure in the ocular fundus has been able to reveal sub-retinal deposits, the optic nerve head, retinal vessels, choroidal vessels, fluid accumulation, hyperpigmentation, atrophy, and breaks in Bruch's membrane—features that have proven difficult or impossible to observe with flood illumination devices. In addition, because infrared illumination is absorbed by the tissues less than other wavelengths, much less illumination from the source is required to create a high contrast image.
The improvements noted above, and methods for successfully imaging small retinal features were combined in U.S. Patent Application No. 60/329,731; Ser. No. 10/493,044; 60/350,836; and PCT Application No. PCT/US02/32787, incorporated by reference herein. In addition, discussions of using the techniques for detecting and localizing such features are described in the publications of the inventor and colleagues: Elsner, A. E., et al., Infrared Imaging of Sub-retinal Structures in the Human Ocular Fundus, Vision Res., Vol. 36, No. 1 (1996), pp. 191-205; Elsner, A. E., et al., Multiply Scattered Light Tomography: Vertical Cavity Surface Emitting Laser Array Used For Imaging Subretinal Structures, Lasers and Light in Opthalmology, (1998); Elsner, A. E., et al., Foveal Cone Photopigment Distribution: Small Alterations Associated with Macular Pigment Distribution, Investigalive Opthalmology & Visual Science, Vol. 39, No. 12 (November 1998), pp. 2394-2404; Hartnett, M. E. and Elsner, A. E., Characteristics of Exudative Age-related Macular Degeneration Determined In Vivo with Confocal and Indirect Infrared Imaging, Opthalmology, Vol. 103, No. 1 (January 1996), pp. 58-71; and Hartnett, M. E., et al., Deep Retinal Vascular Anomalous Complexes in Advanced Age-related Macular Degeneration, Opthalmology, Vol. 103, No. 12 (December 1996), pp. 2042-2053, incorporated by reference herein. The systems and techniques described in the inventor's previous patent applications introduced a moderately priced, portable system that provided a high contrast, digital image of the eye that could be used by non-specialists, such as paramedics or other individuals in the field. However, creating a system that is even less expensive, uses standard digital imaging technology, includes fewer high precision optics to obtain a high contrast image would be greatly appreciated in the art.
In addition, studies have shown that the multiply scattered light images, that are used to reveal structures in the deeper retina, can provide more detailed images that provide additional diagnostic utility. Further, the use of the infrared spectrum can be used to image the retina without dilation of the patient's pupils, and the added potential for using multiply scattered light, particularly in cases in which the target of interest falls below a highly reflective layer, allow visualization of features difficult to see otherwise. However, previous scanning devices, including those embodied in the patent applications submitted by the inventor and her colleagues, do not readily utilize this method for producing an image without scanning not only the light illuminating the target, but also scanning the light returning from the target to the detector, which requires considerable care. Therefore, a moderately priced, portable digital retinal imaging device that is capable of producing multiply scattered light images would be greatly appreciated in the art.
Existing devices specifically designed for screening of retinal disease that use flood illumination with bright lights of shorter wavelengths, and typically acquire single images at slow rates, have been shown recently to provide an unacceptable percentage of gradable images in the hands of technicians (Zimmer-Galler I, Zeimer R. Results of implementation of the DigiScope for diabetic retinopathy assessment in the primary care environment. Telemed J E Health. 2006 April; 12(2):89-98), regardless of the duration of training (Ahmed J, Ward T P, Bursell S E, Aiello L M, Cavallerano J D, Vigersky R A. The sensitivity and specificity of nonmydriatic digital stereoscopic retinal imaging in detecting diabetic retinopathy. Diabetes Care. 2006 October; 29(10):2205-9.) As discussed above, the embodiments of the present application address the issue of inconsistent use in the eye field. Other issues addressed by embodiments of the present application include onboard pre-processing of image and instrument parameter data for quality assurance and ease of use, addressing the issue of alignment of the instrument with respect to the target (e.g., small pupils and addressing and other issues regarding the anterior segment of the eye). The present application further addresses the prior art issue of failing to capture the images of the best existing quality, and failing to operate the instrument with optimal parameters.
Therefore, a moderate cost, portable retinal imaging device that provides for the use of a scanning laser device operating with near infrared illumination and which can allow for multiply scattered light would be appreciated in the art. Further, such a device that would allow for increased ease of use by allowing a greater field of view than just 20 deg visual angle, greater field of view without sacrificing spatial resolution, as well as utilizing a non-proprietary system for producing and saving the digital image, would be greatly appreciated.